1. Field of the Invention
This invention relates to sensing through a check valve housing, hereinafter referred to as the valve body, the position of the internal moveable valving element, hereinafter referred to as the disk, using a transient electromagnetic field to detect the position of an electrically conductive and/or magnetically permeable disk within the valve body which may also be electrically conductive and/or magnetically permeable.
2. Background Information
Check valves are commonly used in fluid systems to limit-flow to one direction. Typically, such check valves have a disk, enclosed within the valve body, which is movable from open to closed positions. In certain applications, for instance nuclear power plants, it is desirable to be able to verily the operation of a check valve. Ideally, this should be accomplished without physical penetration of the valve body or physical dismantling of the valve assembly, i.e., non-invasively.
Electromagnetic check valve inspection systems have been developed for non-invasive monitoring of the position of the internal movable disk in a check valve by applying either low-frequency steady-state alternating current (AC) or fixed direct current (DC) magnetic fields to the external surface of the valve body such that the magnetic field penetrates the valve body and the disk in such a manner that the magnetic field on the external surface of the valve body is influenced by the position of the internal disk. Magnetic sensors are placed at one or more positions in the vicinity of or on the external surface of the valve body to detect changes in the magnetic field at those positions associated with movement of the disk.
The AC methods developed use the basic principles of eddy-current instrumentation in which the applied AC fields result in generation of eddy-currents in the valve structure which in turn result in secondary fields which are those sensed on or near the surface. The characteristics of those secondary fields i.e., magnitude and phase with respect to the primary applied field are sensed with appropriate magnetic field sensors such as coils or Hall-effect devices. At any sensing point, the characteristics of the secondary fields are altered by movement of the disk and those measured changes are calibrated in terms of the position of the disk. See for example; (1) Haynes, H. D., "Check Valves: Oak Ridge's New Diagnostics," Mechanical Engineering, May 1991, (2) Leon, R. L., U.S. Pat. Nos. 5,086,273 and 5,140,263. For valves with wall thickness 1/2" or greater, the frequency of the applied AC field must be very low, typically 60 Hz or lower and/or the strength of the applied field very high, typically 2500 Amp-turns or greater in order to adequately penetrate the valve body of both ferrous and nonferrous valves and induce fields in the moving disk. As the changes in the amplitude and/or phase of the secondary field components associated with movement of the disk are extremely small in comparison with those of the associated with the valve body which is in much closer proximity to the primary field excitation coil and sensors, the measuring system essentially requires taking the difference between two very large signals to extract the desired small signal. This is an error-sensitive measurement technique, a disadvantage of AC methods for this application.
Another drawback with single-frequency eddy-current techniques for valve inspection is that there is limited signal information at only one frequency making it difficult to discriminate the secondary field component associated with the disk from that associated with the large and thick valve body. This situation is analogous to the problem in eddy-current inspection of tubing structures where it is required to detect flaws on the outside of a tube wall using eddy-current probes (coils) inside the tube, the problem further compounded by the presence of tube support structures on the outside. In that and other eddy-current applications, performance is greatly enhanced by using multi-frequency techniques in which two or more frequencies are simultaneously applied to the excitation coil and the sensed signals at each frequency processed to yield information essentially associated with different depths of the structure to enhance discrimination of the defect signal at the desired depth in the overall structure.
The DC methods developed for check valve inspection for disk position sensing are based on variations of classical Magnetic Flux Leakage (MFL) methods used in the non-destructive testing of ferrous materials (see H. D. Haynes reference cited earlier). In this method a DC field is applied to the valve body using either large permanent magnets or DC coils placed externally on or near the valve body and of sufficient magnetic field strength to penetrate the valve body and the disk. DC magnetic field sensors such as Hall-effect devices are placed on or near the valve body at locations suitable to detect the change of the magnetic field associated with movement of the disk. This is similar to the MFL methods for the inspection of large steel billets for the detection of subsurface voids, cracks or other anomalies. Currently, Hall-effect sensors are the only practical sensing means for this valve inspection application taking into consideration cost, complexity, etc. Hall-effect devices are generally low sensitivity devices which poor temperature coefficients of sensitivity and linearity. This is a serious drawback for practical valve inspection. Another disadvantage in the DC methods is that small changes in the relative placement of the magnets (or DC coils), and sensor or sensors, and/or changes in pipe system geometry in the vicinity of the valve will probably dramatically change the functional relationship between the applied primary field and the sensed secondary field associated with the disk position. Furthermore, DC methods can only be applied to ferrous (magnetically permeable) valves, and cannot be used on non-ferrous (stainless steel, brass, etc.) valves.
In recent years, pulsed-eddy current (PEC) techniques have been developed for a wide range of applications requiting eddy-current analysis. Applications include geophysical exploration, metal detection systems and non-destructive testing. See for example; (1) Gibbs, M. et al., "Pulsed Eddy Current Inspection of Cracks Under Installed Fasteners," Materials Evaluation, January 1991, (2) P. F. Lara, U.S. Pat. No. 4,929,896, (3) B. R. Spies, U.S. Pat. No. 4,929,898, (3) S. Linder, U.S. Pat. No. 5,059,902, (4) G. Witrig and H. Thomas, "Design of a Pulsed Eddy-Current Test Equipment with Digital Signal Analysis" American Society for Testing and Materials, ASTM STP 722, 1981, PP. 387-397, (5) R. C. Miller, et al., U.S. Pat. No. 3,707,672.
In PEC methods a pulsed electromagnetic field is applied to the object under test or over the space to be analyzed for a specified period of time to penetrate the object under test or to be detected and then abruptly shut off. At shut-off, the field within the metallic object tries to collapse to zero. As it collapses, however, the time-varying nature of this collapsing field induces eddy-currents in the object which generate secondary fields which try to oppose a change in the original field. The eddy currents and associated secondary fields ultimately decay to zero due to the resistive losses in the material. Appropriate magnetic field sensors (coils, Hall-effect devices, etc.) are used to sense the secondary field. The signal outputs of the sensors are electronically sampled after shut-off of the pulsed excitation field. In conventional PEC methods for various applications, the transient secondary fields generated by the pulsed excitation are sampled, in most cases, at points in time following the field shut-off. The shape, duration and amplitude of the applied pulsed field and the sampling time interval, or intervals, are application specific. The amplitude and temporal characteristics of the post turn-off secondary transient fields are dependent on the geometry, electrical conductivity and magnetic permeability of the metallic object under test and the amplitude and temporal characteristics of the applied pulsed field.
An inherent advantage of PEC methods over steady-state, single-frequency eddy-current methods is that the applied pulse width may be adjusted to assure depth of penetration into metallic structures. Increasing the pulse width is analogous to decreasing the frequency in AC methods. Furthermore, the post turn-off transient fields have time domain waveforms having a wide band frequency spectrum which is dependent upon the electrical properties of the material under test and the time-domain waveform of the applied pulse, both during the on-time and the off-time. Thus, the equivalent of both low and high frequency secondary fields may be generated with a simple pulsed field.
There remains a need for an improved method and apparatus for detecting the position of valving elements in check valves, and especially in check valves which are electrically conductive and/or magnetically permeable. Such a method and apparatus should provide stable reliable results, be simple to implement and operate, be relatively insensitive to changes in piping geometry in the vicinity of the valve and be economically practical for the intended applications.